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POThread Scheduling - Shared Array
 

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    <p>I need two threads to write one a shared array of ints. Both threads need to write on all the elements of that array. Each thread will write either 1 or 7, and the result should be like 171717171 (or 71717171). To do that I have the first Thread1 write at position 0, then wait. Thread2 now writes at position 0 and 1, notifies Thread1, and waits. Thread1 writes at position 1 and 2, notifies Thread2 and waits, etc. With the following code I get correct output, although when run with JPF it finds a deadlock. Its become really frustrating since I can not find whats wrong with it. Any advice would be appreciated. </p> <pre><code>import java.util.logging.Level; import java.util.logging.Logger; public class WriterThreadManager { private int[] array = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0}; private Thread thread7; private Thread thread1; public static void main(String[] args) { WriterThreadManager mng = new WriterThreadManager(); mng.exec(); } public WriterThreadManager() { thread7 = new Thread(new WriterRunnable(this, 7)); thread1 = new Thread(new WriterRunnable(this, 1)); } public void overwriteArray(int pos, int num) { array[pos] = num; printArray(); } private void printArray() { for (int i = 0; i &lt; array.length; i++) { System.out.print(array[i]); } System.out.println(""); } public synchronized void stopThread() { try { this.wait(); } catch (InterruptedException ex) { Logger.getLogger(WriterThreadManager.class.getName()).log(Level.SEVERE, null, ex); } } public synchronized void wakeUpThread() { notifyAll(); } private void exec() { thread7.start(); thread1.start(); } public int length() { return array.length; } } public class WriterRunnable implements Runnable { private WriterThreadManager mng; private int numberToWrite; private static boolean flag = true; @Override public void run() { int counter = 0; int j = 0; //first thread to get in should write only at //position 0 and then wait. synchronized (mng) { if (flag) { flag = false; mng.overwriteArray(0, numberToWrite); j = 1; waitForOtherThread(); } } for (int i = j; i &lt; mng.length(); i++) { mng.overwriteArray(i, numberToWrite); counter++; if (i == mng.length() - 1) { mng.wakeUpThread(); break; } if (counter == 2) { waitForOtherThread(); counter = 0; } } } private void waitForOtherThread() { mng.wakeUpThread(); mng.stopThread(); } public WriterRunnable(WriterThreadManager ar, int num) { mng = ar; numberToWrite = num; } } </code></pre> <p>p.s: an example of the execution:</p> <pre><code>1000000000 7000000000 7700000000 7100000000 7110000000 7170000000 7177000000 7171000000 7171100000 7171700000 7171770000 7171710000 7171711000 7171717000 7171717700 7171717100 7171717110 7171717170 7171717177 7171717171 </code></pre> <p>The error snapshot from JPF is the following: </p> <pre><code>thread java.lang.Thread:{id:1,name:Thread-1,status:WAITING,priority:5,lockCount:1,suspendCount:0} waiting on: WriterThreadManager@152 call stack: at java.lang.Object.wait(Object.java) at WriterThreadManager.stopThread(WriterThreadManager.java:43) at WriterRunnable.waitForOtherThread(WriterRunnable.java:53) at WriterRunnable.run(WriterRunnable.java:45) thread java.lang.Thread:{id:2,name:Thread-2,status:WAITING,priority:5,lockCount:1,suspendCount:0} waiting on: WriterThreadManager@152 call stack: at java.lang.Object.wait(Object.java) at WriterThreadManager.stopThread(WriterThreadManager.java:43) at WriterRunnable.waitForOtherThread(WriterRunnable.java:53) at WriterRunnable.run(WriterRunnable.java:45) </code></pre>
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    1. COIs it a requirement that the writes occur strictly in turn? This seems to be a classic case for using a Semaphore.
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    2. COWell, you synchronize on 3 different objects (the manager, and the two runnables). You don't wait() inside a loop checking for a condition. And every access to the array is not synchronized. Definitely use higher-level abstractions (semaphores, for example) to synchronize your threads.
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    3. COSince this is homework, OP is probably constrained to what he can use.
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      1. USMarko Topolnik
 

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